Connected: An Internet Encyclopedia
RFC 792 - Message Formats

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### Message Formats

ICMP messages are sent using the basic IP header. The first octet of the data portion of the datagram is a ICMP type field; the value of this field determines the format of the remaining data. Any field labeled "unused" is reserved for later extensions and must be zero when sent, but receivers should not use these fields (except to include them in the checksum). Unless otherwise noted under the individual format descriptions, the values of the internet header fields are as follows:

Version | 4
IHL | Internet header length in 32-bit words.
Type of Service | 0
Total Length | Length of internet header and data in octets.
Identification, Flags, Fragment Offset | Used in fragmentation, see [1].
Time to Live | Time to live in seconds; as this field is decremented at each machine in which the datagram is processed, the value in this field should be at least as great as the number of gateways which this datagram will traverse.
Protocol | ICMP = 1
Header Checksum | The 16 bit one's complement of the one's complement sum of all 16 bit words in the header. For computing the checksum, the checksum field should be zero. This checksum may be replaced in the future.
Source Address | The address of the gateway or host that composes the ICMP message. Unless otherwise noted, this can be any of a gateway's addresses.
Destination Address | The address of the gateway or host to which the message should be sent.

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Connected: An Internet Encyclopedia
RFC 792 - Message Formats